A laser system operating in the eye-safe wavelength region is valuable for various applications. Such applications include: laser range-finding systems, laser target designating systems, 3D laser imaging and scanning systems, laser radar devices, communication tools, and other uses that may involve human contact with the laser radiation. Most of the aforementioned applications require high peak power in the megawatt range delivered for 10−7-10−9 nanoseconds. Such a peak power can be obtained via “Q-switch” methods. Q-switching involves adding time-variable losses to the laser cavity, temporarily preventing the laser effect and storing the pumping energy at the excited level of the gain material. After the switch is triggered, the intra-cavity losses are rapidly reduced, releasing the stored energy extremely quickly and generating a very strong laser pulse.
A Q-switch can be either an active device, which is controlled or driven by an external signal, or a passive structure that has no external control, but rather operates as a result of its own properties. Passive Q-switching exploits the non-linear properties of a saturable absorber. A saturable absorber is a material that absorbs radiation at the laser wavelength, and has a low value of saturation intensity. Below a specific threshold, the saturable absorber blocks the incoherent light and prevents laser oscillations from building up in the resonator. At some point, the material becomes transparent due to saturation, and a very strong laser pulse is generated. Passive Q-switching is generally preferred to active Q-switching, due to the simplicity of manufacturing and operation, low cost, and reduced system size and weight.
The common materials in solid state eye-safe laser systems are doped single crystals, glasses and ceramics. These materials each have disadvantages. The glass materials suffer from problems of low thermal conductivity and sensitivity to thermal shock, which result in unstable performance and high risk of fracture under high power laser radiation. In general, the high power operation required in eye safe laser applications necessitates high durability of the materials for the laser system.
Material production of a single crystal is expensive. It is also difficult to uniformly dope the optically active element in a single crystal phase and to produce a finished product large enough for practical purposes. Furthermore, achievable dopant concentrations in a single crystal phase are low, in comparison to a non-crystalline phase.
Glass-ceramics are more cost efficient and more suitable for manufacturing than single crystals. Glass-ceramics consist of a glass matrix and a crystal phase. Such materials can remain transparent like the parent glass (i.e., the glass from which the material is formed), if the crystal size is in the “nano” scale. The dopant ions can enter a crystal phase or a glass phase of glass-ceramics. Several compositions have been proposed for glass-ceramic materials for eye-safe laser applications. Examples include: U.S. Pat. No. 5,483,628 to Borrelli et al, entitled “Transparent Glass-Ceramics”; U.S. Pat. No. 6,197,710 to Ohara et al, entitled “Luminous Glass Ceramics”; and U.S. Pat. No. 6,204,211 to Ohara et al, entitled “Luminous Glass Ceramics”. Significant limitations of laser elements based on glass-ceramics include low optical quality and low laser damage threshold. An additional disadvantage results from light scattering that occurs in the ceramic materials due to the grainy and porous boundaries, causing significant losses of energy.
The passive Q-switch, which was described above, is one type of element in a laser system. Some fluoride based materials have been used as passive Q-switches operating at the eye-safe wavelength region. Examples include Er3+:CaF2, U2+:CaF2, U2+:BaF2, and U2+:SrF2. However these materials have been observed to have a relatively low damage threshold.
Materials based on Co2+ ions doped semiconductors have been tested for use as a saturable absorber for the eye-safe wavelength region (A. V. Podipensky, V. G. Shcherbitsky, N. V. Kuleshov, V. I. Levchenko, V. N. Yakimovich, and V. P. Michailov, “Optics Letters”, Vol. 24, No. 14, 1999, pp. 960-962). However, Co2+ ions doped semiconductors also have a low laser damage threshold and therefore cannot be used in the laser cavity.
Single crystals were also tested for similar use (M. B. Camargo, R. D. Stulz, M. Kokta, and M. Birnbaum, “Optics Letters”, Vol. 20, No. 3, 1995, pp. 339-341; K. V. Yumashev, I. A. Denisov, N. N. Posnov, V. P. Michailov, R. Moncorge, D. Vivien, B. Ferrand, Y. Guyot, “Journal of the Optical Society of America B”, Vol. 160, No. 12, 1999, pp. 2189-2494). One such system is disclosed in U.S. Pat. No. 5,654,973 to Stultz et al, entitled “Laser System Using Co2+-Doped Crystal Q-Switch”. The Q-switching utilizes the properties of the Co2+ ion, which acquires properties of a saturable absorber when it is located in the tetrahedral crystal field. Co2+ doped single crystals, such as Co doped garnets and spinels, have satisfied optical quality requirements and demonstrate relatively high damage threshold. However, growth of a single crystal is a complicated and expensive process.
An alternative technique uses Co2+ doped glass ceramics materials (A. M. Malyarevich, I. A. Denisov, Y. V. Volk, K. V. Yumashev, O. S. Dimshitz, A. A. Zhilin, “Journal of Alloys and Compounds”, No. 341, 2002, pp. 247-250). On the assumption that Co2+ ions enter the crystal phase of the glass-ceramics, the ions can occupy the same location that they occupy in the single crystal. Therefore, the Co2+ ion in the glass-ceramics matrix can have a high absorption cross-section, low saturation fluence, and optimal decay time of the required transition like in the single crystal phase, resulting in high performance of the device. Sufficiently high performance of passive Q-switches based on those materials has been achieved. However, low optical quality and low damage threshold of the glass-ceramics limit their application in the laser systems.
Several compositions have been proposed for a glass-ceramic material suitable for passive Q-switching of eye-safe lasers. Examples include: Russian Federation Patent RU 2,114,495 C1 to Bojko et al, entitled “Passive Q-Switch Material” and USSR Patent SU 1,811,512 A3 to Bojko et al, entitled “Glass for Clear Glass Ceramic Material on Gahnite-Base”.